Add RPC to run constructors

[athei]

Currently, the UI can use the contract_call in order to determine gas costs of a call. However, this is not possible when deploying because there is no RPC that allows running a constructor.

Add a rpc contracts_construct(code_hash, input) -> (ExecResult, ConsumedGas) to allow for gas estimation on deploy.

@jacogr

[ayushmishra2005]

@athei Is below abstraction right?

#[rpc(name = "contracts_construct")]
fn construct(
	&self,
	address: AccountId,
	at: Option<BlockHash>,
) -> Result<Option<ConsumedGas>>;

Please guide me for the implementation part.

[athei]

I am not exactly sure how this will look like because this depends on the changes that will happen to instantiation due to code rent. Adding a new RPC before those are makes no sense.